In line with the increasing use of mobile devices and vehicles, demand for secondary batteries as their energy sources has been rapidly increased. As the secondary batteries, lithium secondary batteries having high energy density, high voltage, long cycle life, and low self-discharging rate have been commercialized and widely used.
A lithium secondary battery may be largely composed of a cathode active material, an anode active material, a separator, and an electrolyte. Specifically, a carbon material has been used as a main component of the anode active material, and in addition, research into using lithium metal, a sulfur compound, a silicon compound, and a tin compound has been actively conducted. Also, a layered structure, lithium-containing cobalt oxide (LiCoO2) has been mainly used as the cathode active material, and in addition, lithium metal compounds having a layered structure (the metal includes manganese, cobalt, nickel, etc.), lithium-containing manganese oxides having a spinel structure (LiMnO2 and LiMn2O4), and lithium-containing nickel oxide (LiNiO2) have been commercialized.
With respect to LiCoO2 which has currently been most widely used among the above cathode active materials due to excellent life characteristics and charge and discharge efficiency, it has limitations in being applied to high-capacity batteries for electric vehicles due to the fact that it has low structural stability, has high raw material costs, and causes environmental pollution. With respect to a lithium manganese oxide, such as LiMnO2 and LiMn2O4, studied as an alternative material of LiCoO2, it has high thermal stability and is inexpensive, but has disadvantages in that electrical conductivity is low, capacity is low, and electrode degradation rapidly occurs at high temperature due to poor high-temperature characteristics. With respect to the lithium-containing nickel oxide, it has battery characteristics of high discharge capacity, but has disadvantages in that it is difficult to be synthesized by a simple solid-state reaction and its cycle characteristics are low.
Therefore, there is an urgent need to develop a novel cathode active material having better high-temperature stability, lower manufacturing costs, and better cycle characteristics than cathode active materials that have been currently used.